There is illustrated in FIG. 1 a construction of this type of wire-dot impact printer adopted conventionally. In the same figure, designated at 100 is a centro I/F, 101 is a CPU, 102 is I/O LSI as an interface, 103 is a timer, 104 is a head driver, 105 is a wire-dot head, 106 is an operation switch, 107 is a line feed motor, 108 is a spacing motor. In the apparatus, the CPU 101 receives a printing date via the centro I/F 100 and supplies a control signal issued on the basis of the printing data to the timer 103, the head driver 104, the line feed motor 107 and the spacing motor 108 via the I/O LSI 102. The head driver 104 receives a control signal from the CPU 101 and a drive timing signal from the timer 103 for driving the wire-dot printing head 105 to effect printing.
As the wire-dot printing head 105, there is an arrangement as illustrated in FIG. 2. In the same figure, designated at 110 are a plurality of printing wires (two printing wires are illustrated in the same figure) provided in the wire-dot printing head 105, 111 is a guide frame having a guide groove llla, 112 is an armature for supporting the printing wires 110, and 113 is a plate spring for supporting the armature 112. Hereupon, designated at 114 is a base plate, 115 is an electromagnet composed of a core 115a and a coil 115b wound around the core 115a, 116 is a permanent magnet, 117 is a rack, 118 is a spacer, 119 is a yoke, and 120 is a clamp. The clamp 120 presses and holds the base plate 114, the permanent magnet 116, the rack 117, the spacer 118, the plate spring 113, the yoke 119, the front cover 111 in a manner such that each of these members are laid one over another in turn and integrated.
The armature 112 is supported at the side of a free end 113a of the plate spring 113 while a base end 110a of one of the printing wires 100 is fixedly mounted on a distal end 112a of the armature 112. A distal end 110b of the printing wire 110 is guided by the guide groove 111a of the guide frame 111 so as to strike a predetermined position of the printing paper (not shown).
With the arrangement as set forth above, when the coil 115b of the electromagnet 115 is deenergized, the armature 112 is attracted to the side of the base plate 114 (downward direction in the figure) by the attraction force of the permanent magnet 116 against the resilience force of the plate spring 113. When the coil 115b is energized, a magnet flux of the permanent magnet 116 is cancelled by the magnet flux of the electromagnet 115 to release the armature 112 from the attraction force of the permanent magent 116 to move the armature 112 toward the side of the guide frame 111 (upward direction in the same figure) by the resilience force of the place spring 113. At the same instant, the printing wire 110 provided at the armature 112 moves toward the side of the guide frame 111 and the distal end 110b thereof projects over the guide slit 111a and strikes the printing paper to effect printing.
FIG. 3 is a circuit diagram of the timer 103 and FIG. 4 is a waveform diagram of the operation of the timer circuit 103. The timer 103 is a portion to adjust an optimum time for energizing the coil 115b on the basis of the voltage to be applied to the coil 115b.
In the same figure, designated at 120 is an open-collector type NOT circuit, 121, 122, 123 are resistors, 124 is a diode, 125 is a capacitor, and 126 is a comparator. The timer circuit 103 operates as follows. Firstly, a signal tl received from the I/O LSI 102 is applied to the NOT circuit 120 on the basis of the instruction from the CPU 101. The signal, t1 becomes high level (5 V) during the period of T1 as illustrated in FIG. 4. At the time when the signal t1 is in high level, an output of the NOT circuit 120 becomes low level (0 V) whereby the electric charge of the capacitor 125 is sharply discharged. After the lapse of the time T1 the signal t1 is returned to low level so that the capacitor 125 is re-charged by a drive power supply voltage Vh which is applied to the wire-dot printing head via the resistor 121 and the output voltage of the NOT circuit 120 increases. The comparator 126 compares a reference voltage Vr which is decided bY the resistance values R122, R123 of the resistors 122, 123 and a power supply Vcc supplied to the logic circuit, which is expressed as R123/(R122+R123) Vcc and the output voltage of the NOT circuit 120. An output signal t2 of the comparator 126 becomes high level during the output voltage of the NOT circuit 120 is lower than the reference voltage Vr while it returned to low level at the time when the output voltage of the NOT circuit 120 reaches the reference compare voltage Vr (after the lapse of time T2). Accordingly, in the case where the drive supply power voltage Vh is high the output voltage of the NOT circuit 120 reaches the reference voltage Vr quickly so that the time T2 when the output of the compartor 126 keeps high level is shortened. In the case where the drive supply power voltage Vh of the wire-dot printing head is low, the time when the output voltage of the NOT circuit 120 reaches the reference voltage Vr in a long period of time, hence the time T2 becomes long.
FIG. 5 illustrates a circuit diagram, of the head driver 104 and FIG. 6 is a waveform diagram of the operation of the head driver 104. In the same figures, denoted at 130 is a buffer gate, 131 is an AND gate, 132, 133, 134 are transistors, 135, 136 are resistors, 137, 138 are diodes, and 115b is the head coil as shown in FIG. 2. The head driver 104 operates as follows. First1y, the buffer gate 130 receives the signal t2 (over drive signal) shown in FIG. 6 from the timer circuit 103 and applies the drive power supply voltage Vh to the head coil 115b. Since the AND circuit 131 receives an enable signal t3 from the timer circuit 103 and a print signal t4 from the I/O LSI 102, the signals t3 and t4 are ANDed at the AND circuit to issue an AND signal to the base of the transistor 134 via a resistor 136. The print signal t4 is a selection signal of the print wire corresponding to the characters to be printed. Accordingly, in the case where all the signals t2, t3 and t4 are high levels, both the transistors 133, 134 will be ON so that the drive power supply voltage Vh is applied to the head coil 115b. Then, the current Ih flows in the direction of the arrow Hl as shown in the one dot one dash line in FIG. 5, and the current value thereof is increased gradually as shown within a range F1 of FIG. 6. In the case where the output of the signal t2 becomes low level after the lapse of time T2, the transistor 133 is OFF so that on the basis of a reverse electromotive force of the head coil 115b a circuit current flows in the direction of the arrow H2 as shown in the two dot one dash line whereby the current value of the current Ih is gradually decreased as shown within a range F2. In the case where the output of the signal t3 becomes low level, the transistor 134 is OFF so that the current Ih flows in the direction of the arrow H3 as shown in the three dot and one dash line and the current value of the current Ih is sharply decreased as shown within a range F3.
In the prior art as described above, in the case where the drive power supply voltage Vh of the wire-dot printing head is high, the time T2 when the signal t2 becomes high level is shortened to thereby shorten the range F1 of the current Ih, while in the case where the drive power supply voltage Vh is low, the time T2 is lengthened to thereby lengthen the range F1 of the current Ih. That is, the current Ih is controlled corresponding to the variation of the power supply voltage VH to be applied to the head coil 115b in order to fix the time of the drive time required from the drive timing for instructing the printing wire 110 to start printing (timing where the signal t1 becomes from the low level to the high level) to the print timing where the printing wire 110 actually strikes the printing paper.
Meanwhile, the drive time from the drive timing to the print timing are differentiated for each print wire by the variation of the interval between the printing wire 110 and the printing medium and magnetic interference of the head coil 115b in the wire-dot printing head 105.
However, in the prior art as described above, although the correction of the variation of the drive power supply volatage Vh of the head coil 115b is made with respect to the drive time of the wire-dot printing head 105, the drive timing for each printing wire 110 is the same and not individually set for each printing wire 110. Therefore, a timing divergence-lag is present between each printing wire 110 thereby producing a displacement of the printing position which results in deterioration of the printing quality.
Furthermore, there was no means for correcting the variation of characteristics of each wire-dot printing head 105 and each printing wire 110 whereby the driving time of the printing wire 110 is not optionally set to optimum for the wire-dot printing head 105 used at that time. In the case where the driving time is less than the optimum value, the energy required for operating the printing wire 110 is small to thereby weaken the striking force of the printing wire 110 against the printing medium to deteriorate the printing quality. To solve, t,he problem, in considering the variation of characteristics each for the wire-dot printing head 105 and the printing wire 110, the driving time is set to be somewhat longer to provide a margin to some extent for the driving time. However, there were problems in that adoption of the step has required much energy for operating the printing wire 110 to thereby, first1y generate much heat in the head coil 115b, and secondly, sometimes a thermal alarm function is operated for preventing the printing head from being highly heated for suspending the operation of the apparatus whereby a throughput is decreased.
Furthermore, a minimum value of the print repetitive cycle due to driving of the wire-dot printing head 105 in the printing process is fixed. That is, a printing speed F (number/sec) (number of printing characters per unit time) in the one line printing operation is gradually increased from the print starting position as illustrated in FIG. 7, maintained at the same speed when it reaches a nominal printing speed Fn, and thereafter decreases gradually at the time close to the print ending position. Accordingly, the orint reoetitive cycle is gradually decreased at the print starting position and is minimum at the constant printing mode and is gradually increased at the print ending position. An optimum value variable in various conditions exists in a minimum value in the printing operation during a prescribed cycle among the print repetitive cycles. For example, in case that the printing medium is one piece of paper, the time taken for actuation operation of the printing wire 110, striking of the the printing medium by the distal end 110b thereof, and returning to the original position of the same (hereafter referred to as a flight time) is a relatively short period. This is caused because the energy when the printing wire 110 struck onto the printing paper is not fully absorbed in the printing paper in case that the printing medium is one piece of paper whereby the printing wire 110 is forcibly bounced due to the resilience force of the platen and the like for supporting the rear of the printing paper. Accordingly, in this case the flight time can be shortened to thereby shorten the print repetitive cycle and increase the printing speed.
However, if the minimum value of the print repetitive cycle is determined in accordance with the flight time of the single paper in the case where coping papers as a printing medium composed of a couple of carbon papers, etc. lay one on another, the coping papers absorb the energY at the time of striking of the printing wire 110 greater than the case of single paper to thereby weaken the elastic bouncing force caused by the platen and the like so that the printing wire returns slowly to its original position. In such case, the flight time is longer, occasionally, than the print repetitive cylce so that the printing wire 110 can not return to its original position before the next printing operation. As a result, it generated such a problem that the, striking energy of the printing wire in the next printing operation is insufficient to thereby considerably deteriorate the printing quality. There was proposed a method for controlling to decide the minimum value of the print repetitive cycle in view of the maximum time of the flight time which varies depending on the kind of printing medium. This method requires that the wire-dot printing head can be used in the large print repetitive cycle which generates such a problem that the printing speed may be reduced than that to be effected by the inherent capacity of the wire-dot printing head.
As another step, a method for controlling to swit-hc the minimum value of the print repetitive cycle in several stages in accordance with the head gap is not a means to solve fully the problem since the flight time is controlled not only by the thickness of the printing medium but the material of the printing medium and also affected by the variation of the characteristic of the wire-dot printing head or variation of the power supply voltage.
Accordingly, it is an object of the present invention to provide a wire dot impact printer to solve the aforementioned problems of the prior art in the manner of preventing the printing from becoming out of position by striking simultaneously a plurality of printing wires onto the printing medium, or correcting the variable of the characteristic for each printing wire, or setting the optimum print repetitive cylce whereby the high qaulity printing can be carried out.